FileDigest.java

package org.andork.io;

import static org.andork.func.ShorterCode.swallowException;

import java.io.IOException;
import java.math.BigInteger;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.security.MessageDigest;

public class FileDigest {
	public static byte[] checksum(Path path, String algorithm) throws IOException {
		try (FileChannel channel = FileChannel.open(path, StandardOpenOption.READ)) {
			MessageDigest digest = swallowException(() -> MessageDigest.getInstance(algorithm));

			ByteBuffer buffer = ByteBuffer.allocateDirect(1024);

			int bytesRead;
			while ((bytesRead = channel.read(buffer)) > 0) {
				buffer.position(0);
				buffer.limit(bytesRead);
				digest.update(buffer);
				buffer.position(0);
				buffer.limit(buffer.capacity());
			}

			return digest.digest();
		}
	}

	public static String format(byte[] checksum) {
		byte[] b2 = new byte[checksum.length + 1];
		System.arraycopy(checksum, 0, b2, 1, checksum.length);
		b2[0] = 0;
		return String.format("%x", new BigInteger(b2));
	}

	public static void main(String[] args) throws Exception {
		Path path = Paths.get(FileDigest.class.getResource("logging.properties").toURI());
		System.out.println(format(checksum(path, "md5")));
	}
}